Apostrophes

An apostrophe (‘) can be used to show that letters are missed out of a word (a contraction).

I am sure I didn’t pick up your pen.

(The two words did and not are joined and the apostrophe replaces the letter o in not.)

An apostrophe can also show ownership or possession. This means that something belongs to someone or something.

The boy’s pen disappeared during an English lesson.

James’ hat had a bobble on the top.

The children’s books were collected in by their teacher.

The girls’ bags were left in the changing rooms after PE.

http://www.oxforddictionaries.com/words/punctuation

http://www.bbc.co.uk/skillswise/topic-group/sentence-grammar